A disc stack centrifugal separator comprises a rotor which is connected to a spindle being driven by a drive arrangement including e.g. an electric motor. Inside the rotor there is a separation space wherein a stack of frustoconical separation discs is arranged. A fluid feed mixture is fed into the separation space and the disc stack, and is separated into at least a light fluid phase and a heavy fluid phase during rotation of the rotor. The light and heavy fluid phases may be continuously led out of the rotor.
The spindle is commonly vertically arranged in a housing of the centrifugal separator. The spindle is journalled in the housing at a first axial end portion of the spindle and the rotor is connected to a second axial end portion of the spindle. The spindle is journalled at the first axial end portion in a bearing arrangement fixedly positioned in the housing. Although the spindle may be journalled at further positions along the spindle, the bearing arrangement is the only journalling of the spindle, which is fixed in relation to the housing.
The spindle extends through two or more spaces of the housing from the bearing arrangement to the rotor. Ideally, the different spaces are sealed from each other and various sealing arrangements have been tried over time. However, three aspects complicate sealing along the spindle of a disc stack centrifugal separator. The spindle with the rotor are rotated at supercritical speed, the above-mentioned fixed journalling at only one axial end portion of the spindle, and the high peripheral speed of the spindle.
Together with the bearing arrangement being fixedly journalled at only one end portion of the spindle, the super critical speed causes a deflection of the spindle. Thus, the spindle is not centred along a straight axis but may deflect up to several millimeters at one or more positions along the spindle. The amount of deflection depends e.g. on centrifugal separator size, rotor weight, and spindle dimensions. The high peripheral speed causes damage to rubber or polymer materials abutting against the spindle. Thus, the use of an elastic sealing element abutting against the spindle and following any deflection of the spindle cannot be used. For instance, an ordinary simmering seal may be used up to approximately 12 m/s peripheral speed. A seal comprising PTFE be used up to approximately 30 m/s peripheral speed, but will not withstand radial movement of the spindle at such high peripheral speed. the peripheral speed at a spindle of a centrifugal separator may be up to 50 m/s or more.
One known type of sealing element used around a spindle between two spaces of a housing of a disc stack centrifugal separator comprises a U-shaped PTFE ring. The PTFE ring extends around the spindle with space between the PTFE ring and the spindle, such that the deflecting spindle does not come into contact with the PTFE ring. Although the narrow space between the PTFE ring and the spindle provides sealing to a certain degree, some transfer of fluid along the spindle between the two spaces of the housing may still occur.